This research program is aimed at the following objectives: a) The study of the molecular and genetic determinants of the interaction between oncogenic DNA viruses and their host cells. These studies will be carried out in vitro, and will use Polyoma and SV40 as model systems, in an attempt to answer the basic question of why these viruses multiply in some cell species while in others they produce neoplastic transformation. b) The study of the molecular basis for host restriction to murine leukemia viruses. These studies will be carried out in vitro on susceptible and nonsusceptible cells infected with mouse leukemia viruses. c) The study of the control of protein synthesis in cells infected or transformed by oncogenic viruses. We will attempt to identify early viral proteins by cell free synthesis. For this purpose the use of viral m-RNA prepared enzymatically as well as of authentic viral m-RNA from infected cells is planned. In addition, we will investigate regulatory mechanisms concerning the level of translation in normal and infected cells.